High-strength steel tubes for high-pressure piping include those of carbon steels, and those of alloy steels containing Si and Mn, and, when necessary, Cr, Mo and/or Al in a low content. For example, a high-pressure fuel tube for an automotive common rail type diesel engine is completed by drawing a tube in a desired size, polishing the inside surface of the tube by electropolishing, chemical polishing, or fluid polishing (abrasive polishing) to ensure pressure tightness, heat-treating the tube by normalizing and annealing, surface-treating the tube by plating or the like for rust prevention, and bending the tube in a predetermined shape.
The high-pressure fuel line of the conventional automotive diesel engine is required to have mechanical properties including an yield point between about 350 and about 500 MPa, a tensile strength between about 500 and 650 MPa, and an elongation between about 22 and 35%. For example, a steel tube having an outside diameter of 6.35 mm and an inside diameter of 3.0 m and capable of being used without undergoing yielding (plastic deformation) is required to have a dynamic pressure rating between about 120 and about 190 MPa under an actual use condition. Practically, the tube has a pressure rating between 100 and 150 MPa counting on safety factor.
Generally, the high-strength steel tube has a sufficient strength for use as an automotive fuel line. However, the recent development of the common rail type diesel engine requires the development of a steel tube having still higher strength for fuel piping.
In a conventional diesel engine, fuel injection valves are connected individually to a fuel pump by fuel lines. In a common rail type diesel engine, a high-pressure fuel supplied by a pump into and accumulated in a common rail interposed between the pump and fuel injection valves, and the high-pressure fuel accumulated in the common rail is distributed to the injection valves respectively combined with cylinders. This common rail type fuel injection system accurately controls fuel injection quantity and fuel injection timing in the entire engine speed range including a low engine speed range and a high engine speed range. Thus the common rail type diesel engine, as compared with the conventional diesel engine, exhibits improved performance, can greatly improve the cleanliness of the exhaust gas, fuel consumption and engine output, and can reduce noise and vibrations.
As the injection pressure of the common rail type diesel engine is raised to cope with the yearly increasing severity of exhaust gas regulations, the need for improving silence, fuel consumption and engine output, fuel injection tubes connected to a common rail are required to have a higher pressure rating.
A technique for enhancing the strength of a fuel injection tube to be used on a common rail type diesel engine is disclosed in Patent document 1. The technique disclosed in Patent document 1 subjects a high-strength steel tube to a heat treatment at 950° C. to form a single-phase austenitic structure, and quenches the high-strength steel tube to a temperature between 350 and 500° C. by an austempering process to enhance the pressure rating and fatigue strength.